The present invention relates to a method and arrangement to detect the entire signal from any photon converting in a detector in an x-ray detecting arrangement comprising a number of spaced apart sensors, said detected photons indirectly creating an amount of free charges proportional to the photon energy.
The first step in the detection of an x-ray is the reaction of the x-ray photon through for example photoelectric effect. In case of photoelectric effect an electron in the detector medium absorbs all the energy of the x-ray photon and this energy is transferred to kinetic energy for the electron. In a semiconductor, e.g. like Silicon or Gallium Arsenide or similar the photoelectron will excite electron hole pairs in the conducting and valence bands. The number of created electron-hole pairs will be proportional to the energy of the photoelectron and thus to the energy of the incident x-ray. The standard way to deduce the energy of the incident x-ray is to measure a signal induced from the electron-hole pairs. In a gas as detector medium the number of electron-ion pairs created by the photoelectron will be proportional to the energy.
There are a number of applications where measurement of the energy of x-rays is important. One can mention x-ray fluorescent spectroscopy and weighting of x-ray photons as a function of energy. In medicine so called dual energy imaging has also been proven useful.
It is obvious that it is necessary to collect the full amount of electrons/holes (or electrons/ions in case of gas) to estimate the energy. If a fraction of the electron-hole pairs are not collected the energy for the x-ray will be underestimated. One way of losing electron holes is if they diffuse to adjacent detector pixels before they are collected. This will occur for x-rays converting close to the borderline between two or more detector pixels.
In some applications there are demands on high spatial resolution corresponding to small pixels and high-energy resolution. Since small pixels increases the risk for charge sharing this is in contradiction with obtaining high-energy resolution.
U.S. Pat. No. 5,665,969 describes a x-ray detector, designed to operate as an imaging spectrometer for imaging of a subject. The x-ray detector measures energy of individual x-ray photons in each of a plurality of pixels in the x-ray detector. The pixels of the x-ray detector are readout at a rate such that the likelihood for arrival of more than one x-ray photon in each pixel during a readout period is negligible. Because x-ray photons with different energy levels will create different magnitude responses in the x-ray detector, the measurements made by the x-ray detector can be weighted according to the energy level of the detected x-ray photons. Thus, responses due to noise or x-ray photons, which contribute little or no x-ray attenuation information, can be discarded or weighted to eliminate or reduce their effect on any resulting image. Conversely, measurements due to x-ray photons, which provide significant attenuation information, can be weighted significantly.
According to this patent, the optimal energy weighting one should use is the theoretical optimal one that is approximately proportional to the negative third power of energy. There is nothing mentioned about charge sharing and the optimal weight curve, which can be used in reality.
EP 0 767 389 discloses an X-ray image pickup device having a two-dimensional image reading device, which is constituted by two-dimensionally forming a plurality of photoelectric conversion elements on an insulating substrate. It also has switching elements in the vicinities of these photoelectric conversion elements, a phosphor which is formed on the two-dimensional image reading device and serves as a wavelength converter for converting X-ray radiation into visible light. A grid plate is arranged, which is formed on the phosphor and guides only X-rays from a specific direction toward the phosphor and the two-dimensional image reading device.
A three terminal solid-state ionizing radiation detector, according to U.S. Pat. No. 5,627,377, includes a first layer of a substantially intrinsic Group II-VI compound semiconductor material, such as CdZnTe. The first layer is responsive to incident ionizing radiation for generating electron-hole pairs. The detector further includes a second layer of Group II-VI compound semiconductor material and a third layer of Group II-VI compound semiconductor material that is interposed between first surfaces of the first layer and the second layer. The third layer functions as a grid layer. A first electrical contact is coupled to a second surface of the first layer, a second electrical contact is coupled to a second surface of the second layer, and a third electrical contact is coupled to the third layer for connecting the detector to an external circuit that establishes an electric field across the detector. The electric field causes holes to drift away from the grid layer towards the first contact while electrons drift towards and through the grid layer, through the second layer, and towards the second contact for generating a detectable output signal pulse. Because of the presence of the grid layer only the electrons contribute to the output pulse. The grid layer has a conductivity type such that electrons are a minority charge carrier within the grid layer.
Examples of detectors, on which the present invention can be applied to are disclosed in U.S. Pat. No. 4,937,453 and the pending Swedish Patent Application No. 9900856-7. U.S. Pat. No. 4,937,453 discloses a method and apparatus for detecting x-ray radiation in a radiographic imaging context using so-called xe2x80x9cedge-onxe2x80x9d detectors. This detector is particularly useful in conjunction with slit and slot scan radiography. In accordance with this invention, detectors are constructed and arranged such that substantially all of the energy from an X-ray to be detected is discharged in the detector. In this way a detector is provided which provides a direct electronic read out, high X-ray stopping power and high spatial resolution while obtaining good signal collection efficiency without the use of excessively high voltage levels. In the preferred embodiment, solid state x-ray detectors are constructed such that the thickness of the detector along the direction of incident x-rays is long enough that substantially all of the x-ray energy is discharged in the detector.
The application No. 9900856-7 refers to a method of obtaining improved radiographic images consisting of orienting a semiconductor radiation detector. The orienting step comprises a selection of an acute angel between the direction of incident radiation and a side of the detector such that the incident radiation mainly hits the side.
FIG. 1 is a schematic illustration of a detector 100 comprising a semiconducting substrate 110 and spatially arranged sensor or electrode strips 120.
Common for these detectors is that stripes of sensors are arranged spaced apart on a silicon substrate and the x-rays being incident onto both the sensors and the space between them.
Swedish Pending Patent Application No. 9903655-0, by the same applicant, relates to a method of detecting a number of photons in an x-ray detecting arrangement comprising a number of spaced apart sensors, said detected photons indirectly creating an amount of free charges proportional to the photon energy. The method comprises weighting said photons by means of signals with respect to possible photon charge-share between at least two adjacent sensors.
It is the main objective of the present invention to enhance the prior art methods by eliminating charge sharing entirely or partly.
Yet, another essential object of the invention is to present a way to simultaneously obtain high spatial resolution and high-energy resolution for incident x-rays on x-ray detector. Another object of the invention is to extend the photon path in the collimator to absorb the substantially the entire photon energy.
For these reasons the method according to the invention comprises arranging a blocking means between said spaced apart sensors for stopping x-rays that will be incident close to edges of said sensors and will give rise to charge sharing between adjacent sensors. The method further comprises arranging said blocking means directly over the space of said spaced apart sensors or in a distance over the space of said spaced apart sensors. Preferably, the blocking means comprises a blocking material, such as wolfram, copper or any other element or compound with a reasonably high stopping power for x-rays. The blocking means can be produced through etching, or it comprises etched layers adhered together to a blocking layer.
In an assembly for detecting and counting photons in x-ray beams, the invention relates to an arrangement when detecting substantially the entire signal from said photons converting in a detector in an x-ray detecting arrangement comprising a number of spaced apart sensors, said detected photons indirectly creating an amount of free charges proportional to the photon energy. The arrangement comprises a blocking means between said spaced apart sensors for blocking charge sharing between the sensors. Preferably, the blocking means is directly arranged over the space of said spaced apart sensors. It may also be a grid arranged in a distance over the space of said spaced apart sensors. The blocking means may be made of a blocking material, such as wolfram, copper or any other element or compound with a reasonably high stopping power for x-rays. Preferably, said blocking means is produced through etching or it comprises etched layers adhered together to a blocking layer.